I have taken serveral different Diet pills with no results, WHY?

I have taken since Nov 2012 under MD supervison Phentermine HCL 37.5mg, Phendimetrazine Tart 105 mg, Phendimetrazine Tart 35mg 2 tablets twice daily, Phentermine HCL 30 mg with NO results of wt loss. Now he has prescribed Diethylpropion 75mg ER, what are the chances that this will work?

I took the phentaramine a few yrs ago & it worked amazingly but since i'ce gottn fibromyalgia i've gained weight & have tried it again a few more time & have had no luck. What were the other pills u had like??